1. Field of the Invention
The present invention relates generally to distortion correction systems and, in particular, to the automatic correction of distorted computer projection display images.
2. Description of the Prior Art
Unless a projector is carefully aligned to the projection surface (screen), the resulting image on the screen appears distorted, or xe2x80x9ckeystonedxe2x80x9d. In prior art, xe2x80x9ckeystonexe2x80x9d refers specifically to a symmetric, trapezoidal distortion caused by projector pitch misalignment. In this application, the term xe2x80x9ckeystonexe2x80x9d refers to the broader class of distortions caused by any misalignments in projector position or orientation.
Keystoning is undesirable, not only because viewers find this warping to be distracting, but also because the distortion can significantly affect the interpretation of visual information such as graphs, bar charts and technical drawings. This keystoning can be prevented by aligning the projection system""s optical axis so that it is perpendicular to the screen and ensuring that the image is not rotated with respect to the screen.
Unfortunately, in many situations, these constraints (particularly the former) create a projected image that is either too small (when the projector is too close to the screen) or badly placed (when the projected image is too close to the floor). In view of these constraints, keystoning is often unavoidable.
Some sophisticated LCD projectors offer manual keystone correction.
Using manual controls, a user is able to adjust the projection system to eliminate distortions caused by vertical misalignment. The prior art only addresses symmetric vertical keystoning caused by projector pitch (where a rectangular source image projects to a trapezoid on the screen). In typical projectors, a pitch of up to xc2x112xc2x0 may be rectified and the best systems can correct a pitch up to xc2x120xc2x0. More importantly, the prior art cannot rectify distortions caused by any other misalignments, such as projector yaw or roll.
The prior art requires the user to manually adjust the projection system to eliminate keystoning. This can be time consuming and requires the user to have access to the projector. Manual controls would be particularly limiting if the projector were capable of rectifying a broad class of distortions since the user would be required to simultaneously optimize several parameters.
It is, therefore, an object of the present invention to exceed the prior art limitations of pitch correction. It is another object of the present invention to allow rectification of any distortions due to projector placement and enable users to mount projectors anywhere in the room. It is a further object of the present invention to enable fully automatic adjustment of all parameters. However, if the user wishes to manually refine the process, the present invention also offers an intuitive interactive interface.
The method of the present invention entails determining a mapping between points in the source image frame and the corresponding points in the viewer image frame; identifying boundaries of the projected image frame within the viewer image frame; computing a possible mapping between the projected image frame and the viewer image frame; inferring a possible mapping from the source image frame to the projected image frame based upon the mappings computed in the previous two steps; determining an optimal placement for the corrected application image within the projected image frame (this is the largest rectangle that is completely contained within the projection of the computer display, i.e., the keystoned quadrilateral); computing a pre-warping transformation based on the previous two steps; pre-warping each application image to correct for keystoning based on the pre-warping transformation; and continuously recalibrating the system if desired (assuming that computational resources are available).
Further, the present invention includes an apparatus that is capable of performing the above-described methodology. In this application, the more general problem of aligning the corrected image to a physical projector screen is discussed. The simpler problem of aligning the corrected image to the camera image can be solved by equating the camera image to the physical screen.